The invention relates to a selective trip unit associated with a limiting circuit-breaker protecting an alternating current system or installation.
In a trip unit of the kind mentioned, the number of beats or contact opening and closing cycles occurring on a circuit fault is counted and integrated either by a mechanical counter, in particular a cog-wheel moving notch by notch, or by a digital or analog electronic counter. The limiting circuitbreaker is fitted with a standard thermal or thermomagnetic type trip unit.
The association of such an intrinsical trip unit with an electronic counter of the number of successive cycles gives rise to problems of dimensions, as the electronic counter requires specific operation control parts, i.e. a current transformer with two secondary windings, a supply unit, etc. . . Another technical problem is linked to the operation of the electronic counter which records a pulse or fault current each time the line current exceeds a given threshold, the value of which is lower than the current causing fast opening of the contacts. It has been noted that the peak current intensity of the measurement signal delivered by the current transformer associated with the line to be protected is not the exact image of the effective value of the prospective short-circuit current. Identification of the fault current pulses corresponding to the opening closing cycles of the limiting circuit-breaker contacts is imperfect with such a device, and a faulty or spurious trip can occur as a result.
The object of the present invention is to avoid these drawbacks and to enable a discrimating solid state trip unit to be produced with reduced overall dimensions and a high degree of short-circuit protection reliability whatever the shape and state of the fault current.